Bone loss diseases are currently a health threat for approximately 44 million Americans, including 10 million with osteoporosis and 34 million with low bone mass and at risk for developing osteoporosis. The number of Americans with osteoporosis is expected to rise by 2020. Consequently, a large number of individuals are at risk for bone fracture due to low bone mass. Approximately 40% of white women and 13% of white men over age 50 are at risk for hip, spine, or forearm fracture within their lifetime. The costs associated with osteoporosis-related fractures were approximately $18 billion dollars in 2002, and are expected to continue climbing. In addition to primary (age-related) osteoporosis, disuse osteoporosis is an important clinical problem, especially for patients chronically immobilized due to stroke or spinal cord injury. Fracture rates double compared to healthy controls in the first year following spinal cord injury and are also elevated compared to healthy controls after the onset of stroke. Disuse increases fracture rates primarily because reduced skeletal loading causes unbalanced bone remodeling which leads to bone loss.